1. Field of the Invention
The present invention relates to a combustion apparatus for a heating apparatus used for a heater, hot water supply system, air conditioner, and the like.
2. Related Art of the Invention
In place of the conventional combustion apparatus forming flame, a catalytic combustion apparatus has been proposed in which the emission of nitrogen oxides is greatly reduced and the exhaust gas is made clean. However, if the catalytic combustion apparatus is operated with the same combustion intensity (combustion amount per combustion chamber volume) as that of the flame combustion apparatus, the catalyst body temperature reaches 1200.degree. C. or higher, exceeding the heat resistance limit of catalyst, so that the service life is decreased remarkably.
As a means to solve this problem of combustion intensity, as shown, for example, in one embodiment of Japanese Patent Application No. 7-316888 shown in FIG. 16. a combustion system is available which is made up of a first catalytic combustion section 104 having a system for simultaneously carrying out combustion and heat exchange and a second catalytic combustion section 112 having a honeycomb catalyst body 114 provided on the downstream side of the first catalytic combustion section 104. The fuel mainly burns while performing heat exchange in the first catalytic combustion section 104, so that the temperature does not rise unlike the flame combustion, and naturally flame is not formed. The remaining lean fuel is catalytically burned in the second catalytic combustion section 112 on the downstream side. Here is utilized the advantage of catalytic combustion combustible even if the fuel is lean. The first catalytic combustion section 104, which uses the high heat transfer characteristics of combustion of a catalyst 107, is provided with the catalyst body 107 in the vicinity of a heat receiving fin 105, constituting a heat exchange type catalytic combustion section. The water in a cooling path 108 turns to warm water in the first catalytic combustion section 104 and an exhaust heat recovery section 106. Since the heat receiving fin 105 for heat exchange is directly covered by the catalyst body 107, the heat transfer rate of generated heat in the catalyst 107 to the heat receiving fin 105 is high. Therefore, this system is a compact and high-efficiency heat exchanger integrated combustor.
When combustion is started, it is necessary to heat the catalyst beforehand to the reaction temperature or higher. For this purpose, there is proposed a method in which flame is formed before the start of catalytic combustion or a method in which the first catalytic combustion section and second catalytic combustion section are preheated by an electric heater before the start of combustion.
The present invention was made to solve the following two problems with a conventional two-stage catalytic combustion apparatus of this type.
1. Stabilization of first-stage catalytic combustion and increase in endurance life
In the two-stage catalytic combustion apparatus, the operation of cooling in the combustion section is essential because combustion takes place with the same air-fuel ratio as that of flame combustion and at a temperature lower than the heat resistance limit temperature. However, it is difficult to stably perform the inconsistent operation in which cooling is performed while heat is stably generated by a catalyst. Under certain conditions, if the heat generation is excessive in the first-stage catalyst, the first-stage catalyst temperature increases dramatically, so that the heat resistance limit temperature is exceeded. If cooling is excessive, the combustion reaction at the first stage decreases, so that high-concentration unburned gas slips to the downstream side. When the second-stage catalyst exists, the combustion therein becomes excessive, so that the second-stage catalyst exceeds the heat resistance limit temperature. To prevent these phenomena, it has conventionally been necessary to accurately control the air-fuel ratio and other conditions, and the stable combustion of the first-stage catalyst, which is the main combustion, has been demanded to make the combustor more excellent.
Generally, the reactivity of catalyst is sometimes decreased by the long-term use and the service conditions. To improve the practicality, it is important to prevent this phenomenon. For the catalyst for combustion, the use at a temperature below the heat resistance temperature is essential. The heat resistance temperature, differing depending on the type of catalyst and the like, is said to be approximately 900.degree. C. for a precious metal catalyst normally used for combustion. Because this is characteristic of catalyst material, how to use catalyst is a point. From this viewpoint as well, it is found that it is an important problem to stabilize the first-stage catalytic combustion by properly setting the heating and cooling in the combustion section and to maintain the catalyst temperature at a proper value.
2. Improvement in thermal efficiency and improvement in energy saving
Thermal equipment is always required to have a high thermal efficiency. The most important point for meeting this requirement is that the heat dissipation loss is at a minimum, so that it is desired to reduce heat dissipation loss caused by the convection from the body surface. Conventionally, a method of covering the surface with an insulating material has been used. However, this method is contrary to the trend of downsizing of equipment. Also, because the catalytic combustor has a construction in which elements are packed in the combustion chamber, it is difficult in the catalytic combustion system to use a configuration such that a water passage is arranged around the combustion chamber, this configuration being often used in the flame combustion system.
Further, in the catalytic combustion, it is necessary to heat the catalyst beforehand to the activation temperature or higher in order to start combustion reaction, so that a preheating means such as an electric heater is often used. However, there arises a problem in that preheating takes much time because the heater itself has a heat capacity, and the rise time is longer than that of the flame combustion. The long rise time caused by the use of electric heater for preheating leads to an increase in electric power consumption, with the result of impaired energy saving.